Process for the preparation of polyesters having stabilized melt viscosity in the presence of aromatic (poly-)carbonates

ABSTRACT

The melt viscosity of high molecular weight linear polyesters is stabilized in the presence of aromatic (poly-)carbonates by adding a minor proportion of a non-volatile monofunctional ester-forming compound to a mixture of glycol and terephthalate or isophthalate reactants and heating the resulting mixture. The products of the process are suitable for conversion to compositions with aromatic (poly-)carbonates, especially flame retarded such compositions, where stabilized melt viscosity during fabrication is critical.

CROSS-REFERENCE TO RELATED APPLICATION

This is a division of application Ser. No. 751,365, filed Dec. 16, 1976,now U.S. Pat. No. 4,126,592.

This invention relates to the production of thermoplastic polyestercompositions. More particularly, it pertains to stabilizing the meltviscosity of linear high molecular weight polyesters by adding amonofunctional ester-forming compound to the difunctional esterificationreactants and heating the mixture prior to compounding with aromatic(poly-)carbonates. The products of the process are superior forprocessing where melt stability is critical, especially when modifiedwith flame retardant additives comprising aromatic (poly-)carbonatesand/or synergists, e.g., antimony compounds.

BACKGROUND OF THE INVENTION

High molecular weight linear polyesters and copolyesters of glycols andterephthalic or isophthalic acid have been available for a number ofyears. These are described inter alia in Whinfield et al, U.S. Pat. No.2,465,319 and in Pengilly, U.S. Pat. No. 3,047,539. These patentsdisclose that the polyesters are particularly advantageous as film andfiber-formers.

For certain applications, such as injection or extrusion, and blowmolding or blown film extrusions, it is desirable to use polyesterresins of high and stable melt viscosity. Linear polyesters, however,are known to interact with aromatic carbonates and (poly-)carbonatesduring compounding to give very high molecular weight/melt viscosityproducts.

Such carbonate species may include, e.g., decabromodiphenyl carbonate,copolycarbonates of bisphenol-A and tetrabromobisphenol-A, bisphenol-Apolycarbonate, tetrabromobisphenol polycarbonte, and the like. Increasein melt viscosity is particularly undesirable when the polyester iscompounded with bisphenol-A (BPA)-tetrabromo BPA copolycarbonate (flameretardant), and Sb₂ O₃ (synergistic flame retardant additive) that,unavoidably, also acts as catalyst in the linear polyesterpolycarbonatereaction. See, U.S. Pat. Nos. 3,936,400; 3,833,685; 3,833,535;3,855,277; and 3,915,926. The interaction causing increases in meltviscosity proceeds via terminal OH groups on the polyester, e.g.,poly(1,4-butylene terephthalate), molecules. It has now been discoveredthat by blocking these with monofunctional reagents of low volatility,so that they survive the polyester manufacturing process, theinteraction with aromatic carbonate is blocked and melt viscosity of theblends can be kept under control.

DESCRIPTION OF THE INVENTION

According to this invention, there is provided a process for producing ahigh molecular weight linear polyester resin which is melt viscositystabilized in the presence of an aromatic carbonate or (poly-)carbonate,said polyester resin being selected from the group consisting ofpolymeric glycol terephthalate and isophthalate esters having repeatingunits of the general formula: ##STR1## wherein n is a whole number offrom 2 to 10, and mixtures of such esters, said process comprisingadding a small amount of a relatively non-volatile monofunctionalester-forming compound to an esterification mixture comprising thecorresponding glycol and terephthalic or isophthalic acid or a reactivederivative thereof and heating the mixture under polyesterificationconditions until formation of said melt viscosity stabilized polyesteris substantially complete.

When used herein, the expression "stabilized melt viscosity" refers tothe flowability of the molten resin in intimate admixture with anaromatic carbonate or (poly-)carbonate relative to the resin alone. Itis a property which can be measured by means well known to those skilledin the art. One convenient method is the use of a capillary meltviscometer known as "Melt Strength Tester" of the type manufactured byToyo Seiki, Tokyo, Japan. The method will be described hereinafter.

The higher molecular weight linear polyesters formed in the presentprocess are polymeric glycol esters of terephthalic acid and isophthalicacids. They can be prepared by known techniques such as by thealcoholysis of esters of the phthalic acid with a glycol and subsequentpolymerization, by heating glycols with the free acids or with halidederivatives thereof, and similar processes. These are described in U.S.Pat. Nos. 2,465,319 and 3,047,539, and elsewhere. In addition to thephthalates, amounts, e.g., from 0.5 to 15% by weight, of other aromaticdicarboxylic acids, such as naphthalene dicarboxylic acid, can bepresent in the polyester component. Although the term "linear" is used,the reactants can also include amounts of tri- or polyfunctionalbranching agents, such as trimethylolpropane, pentaerythritol, trimethyltrimesate. In any event, however, all such processes are modifiedaccording to this invention by including in the polyesterificationmixtures, small amounts of relatively non-volatile monofunctionalester-forming organic compounds.

Preferred polyesters will be of the family consisting of high molecularweight, polymeric glycol terephthalates or isophthalates havingrepeating units of the general formula: ##STR2## wherein n is a wholenumber of from 2 to 10, preferably from 2 to 4, and mixtures of suchesters, including copolyesters of terephthalic and isophthalic acids ofup to about 30 mole % isophthalic units.

Especially preferred polyesters are poly(ethylene terephthalate) andpoly(1,4-butylene terephthalate). Special mention is made of the latterbecause it crystallizes at such a good rate that it may be used forinjection molding without the need for nucleating agents or long cycles,as is sometimes necessary with poly(ethylene terephthalate).

Illustratively, after completion of the reaction, the high molecularweight polyesters will have an intrinsic viscosity of about 0.6 to 2.0dl./g. and preferably, from 0.7 to 1.6 dl./g., as measured, for example,in a 60:40 phenol-tetrachloroethane mixture at 30° C.

Non-volatile monofunctional ester-forming organic compounds selected foruse can vary within fairly wide limits. Preferably, the compounds willcontain only carbon, hydrogen and oxygen. The compounds preferably willhave a molecular weight of below about 3500, to facilitate blending withthe glycols and terephthalate and/or isophthalate and or reactivederivatives thereof, used in forming the polyester resin units. Themonofunctional group is a carboxyl group, a carboalkoxyl group or analcohol group. Especially useful compounds include oleic acid, methyloleate, stearic acid, methyl stearate, palmitic acid, methyl palmitate,octadecyl alcohol, hexadecyl alcohol, and the like. Other especiallyuseful such compounds are a methyl ester of hydrogenated wood rosin,available from Hercules, Inc., Wilmington, Delaware, under the tradename"Hercolyn D"; abietol, a partially hydrogenated alcohol obtained byreduction of wood rosin, also available from Hercules; and a fullyhydrogenated wood rosin, available from Hercules under the tradename"Foral AX".

The monofunctional ester-forming compounds are readily available or canbe made in ways known to those skilled in the art.

The monofunctional ester-forming organic compound may be employed withthe ester resin forming ingredients in any effective amount, butpreferably, small amounts are used, e.g., at a range of 0.1 to 15% byweight. However, a particularly preferred range is 0.5 to 12% by weight.Within this particularly preferred range, it has been found advantageousto employ in certain compositions about 0.5 to 10% by weight. Allpercentages are based on the combined weights of glycol, terephthalate,reactive derivative thereof, etc.

The process of this invention can be carried out in standard proceduresfor making high molecular weight polyesters. In one convenient way, areactor is charged with the terephthalic acid and/or isophthalic acid oran alkyl ester thereof and the glycol, preferably a small excess, anoptional catalyst, e.g., a tetraalkyl titanate, especially if esterreactants are used, and the monofunctional ester-forming compound areadded. The mixture is then heated, e.g., to between 150° and 220° C.under a moderate vacuum, e.g., 0.2 to 5 inches of mercury, until waterand/or methanol and excess glycol ceases to be evolved and production ofa polyester-prepolymer is substantially complete. The mixture is thentransferred to a suitable reactor and heated, e.g., to between 190° and275° C. under a very high vacuum, e.g., about 0.2 mm of mercury, untilthe desired degree of polymerization is achieved. Alternatively, theprepolymer can be made without the addition of the monofunctionalester-forming compound, then the prepolymer can be intimately blendedwith the monofunctional ester-forming compound, and the intimate blendcan be heated to high temperature under a high vacuum until a resin ofthe desired degree of polymerization is obtained. In any case, themolecular weight can be elevated still more by the known technique of"solid state polymerization" which comprises heating subdivided solidresin at a temperature above its softening point, but below its stickingpoint until the desired degree of increase is achieved. Illustrativetemperatures are generally in the range of 150° to 215° C.

In a preferred feature of this invention, a flame retardant additivecomprising an aromatic (poly-) or (copoly-)carbonate will be added tothe melt viscosity stabilized polyesters prepared according to thisinvention, and the resulting compositions will have substantiallyimproved processability.

Illustrative flame retardant additives of this type are disclosed inU.S. Pat. No. 3,915,926 and 3,671,487 which are hereby incorporated byreference. They can be used alone or admixed with synergists, such asantimony compounds, e.g., antimony oxide.

The amount of aromatic (homopoly-) or (copoly-)carbonate flame retardantadditive used is not critical to the invention, so long as it is presentin a minor proportion based on said composition--major proportions willdetract from physical properties--but at least sufficient to render theblock polyester resin non-burning or self-extinguishing. Those skilledin the art are well aware that the amount will vary with the nature ofthe resin and with the efficiency of the additive. In general, however,the amount of additive will be from 0.5 to 50 parts by weight per 100parts of resin. A preferred range will be from abut 3 to 40 parts and anespecially preferred range will be from about 8 to 40 parts of additiveper 100 parts of resin. Synergists, e.g., antimony oxide, will be usedat about 2 to 10 parts by weight per 100 parts of resin.

Among the typical flame retardant additives are those consisting ofaromatic (homopoly-)carbonates having repeating units of the formula:##STR3## wherein R¹ and R² are hydrogen, (lower)alkyl or phenyl, X¹ andX² are bromo or chloro and m and r are from 1 to 4. These materials maybe prepared by techniques well known to those skilled in the art.Preferred are aromatic (copoly-)carbonates in which from 25 to 75 weightpercent of the repeating units comprise chloro- or bromo-substituteddihydric phenol, and the balance are unsubstituted dihydric phenol,e.g., bisphenol-A units. See, e.g., A. D. Wambach, U.S. Pat. No.3,915,926, above-mentioned.

The additives can be intimately blended in a number of procedures. Inone way, the flame retardant additive is put into an extrusioncompounder with the dry polyester resin and the blend is heated at anelevated temperature, e.g., 450°-550° F., and extruded to producemolding pellets. The additive compound or compounds are dispersed in themolten polyester resin by the process. In another procedure, the flameretardant compound(s) is mixed with the polyester resin by blending atordinary temperatures, then the blend is fluxed on a mill, heated, e.g.,at 450°-550° F., then cooled and comminuted; or the blend can beextruded at 450°-550° F., cooled and chopped. The flame retardantcompound(s) can also be mixed with the powdered or granular polyesterand the mixture can be heated and directly formed into blow molded itemsusing machines which compound and mold.

It is always very important to thoroughly free all of the ingredients:resin and additives from as much water as possible before blending them.

In addition, compounding should be carried out to ensure that theresidence time in the machine is short; the temperature is carefullycontrolled, the friction heat is utilized; and an intimate blend betweenthe resin and the additive is obtained.

It should be understood that the polyesters reacted with themonofunctional compounds by this invention are useful as melt viscositystabilized components in the presence of aromatic (poly-)carbonates infurther combination with other conventional additive agents such as, forexample, antioxidants, carbon black, reinforcing agent, plasticizers,lubricity promoters, color stabilizers, ultraviolet absorbers, X-rayopacifiers, dyes, pigments, fillers, and the like.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

The following examples illustrate the invention. They are set forth as afurther description but are not to be construed as limiting theinvention thereto.

The melt viscosity of the compounded polyesters are measured in acapillary melt viscometer ("Melt Tension Tester Model II") built by ToyoSeiki Seisaku Sho, Tokyo, Japan. The instrument consists of a heatingcylinder and orifice of the same dimensions as the "melt indexer"described in ASTM-D 1238. After preheating the polymer example, it isforced through the capillary orifice by means of a piston, driven bymechanical means at a constant rate chosen to provide a shear rate atthe wall of the capillary of 10 sec.⁻¹. The melt temperature during thedetermination is maintained at 250° C. The force applied to the pistonis measured with a load cell and recorded continuously on a strip chart.The melt viscosity is calculated from the force on the piston and theshear rate at the wall of the capillary. In general, the melt viscosityis observed to change with time; the rate of change over a time periodt₁ to t₂ minutes from the start of the measurement is calculated fromthe formula: ##EQU1## in which M₁ and M₂ are the melt viscositiesdetermined after t₁ and t₂ minutes heating time.

EXAMPLE 1

Dimethyl terephthalate, 30 lbs., 1,4-butanediol, 25 lbs., and 11 g. oftetra(2-ethylhexyl)titanate, are mixed with 100 g. of stearic acid andheated in stages to 202° C. and the vacuum is increased to 0.3 in. Hg.The prepolymer is transferred to a high vacuum reactor andpolymerization is carried out at about 250° C. and at a pressure ofabout 0.2 mm of Hg. After two hours, the vacuum is released and thepolyester resin product has a melt viscosity of 7,500 poises at 250° C.

The polyester, 1,033 g., is compounded with a flame retardant agentcomprising a 1:1 copolycarbonate of bisphenol-A and tetrabromobisphenol-A (prepared according to the general procedure of U.S. Pat.No. 3,833,685), 75 g. of antimony oxide and 2.25 g. of antioxidant(Irganox 1093), using a 11/2" extruder, to produce a flame retardantcomposition. The extrudate is pelletized and dried then placed in acapillary melt viscometer and the melt viscosity determined over aperiod of from 5-10 minutes after loading the sample in the viscometerat 250° C., γ=10 sec.⁻¹. The composition according to this invention hasa viscosity of 14,850 poises after 5 minutes, which gradually decreasesto 10,800 poises after 15 minutes, corresponding to a rate of decreaseof 2.7% per minute.

EXAMPLE 2

The procedure of Example 1 is repeated substituting 200 g. of stearicacid for the 100 g. used as the non-volatile monofunctionalester-forming compound according to this invention. After polyesterformation is substantially complete, the polyester has a melt viscosityof 1,830 poises. This indicates that a higher level of stearic acidproduces a lower molecular weight, as would be expected from a molecularweight control additive.

After compounding 1,033 g. of the polyester resin product according tothis example, with the flame retardant additives and antioxidants, thefinal product has a melt viscosity of 4,220 poises after 5 minutes,increasing to 4,530 poises after 15 minutes (the sample is measured witha small orifice at γ=376 sec.⁻¹), at a rate of increase of 0.7% perminute.

EXAMPLE 3

Poly(1,4-butylene terephthalate) is prepared by the method of Example 1except that 1.1 weight percent oleic acid based on dimethylterephthalate is added to the charge. This resin (PBT) is compoundedinto a flame retardant composition as follows:

    ______________________________________                                        Composition          Parts by Weight                                          ______________________________________                                        PBT resin            66.8                                                     Brominated (poly-)carbonate                                                                        26.0                                                     Antimony oxide       5.0                                                      Irganox 1093, stabilizer                                                                           0.15                                                     Ferro 904, stabilizer                                                                              0.05                                                     Bisphenol-A (poly-)carbonate                                                                       2.0                                                      ______________________________________                                    

The melt viscosity of the composition from oleic acid-modified resin is8900 poise after 5 minutes and 6680 poise after 15 minutes, a decreaseof 2.5%/minute, indicating that no interaction takes place between PBTand poly-carbonate. In contrast, the melt viscosity of a PBT which hasnot been modified with oleic acid is 9290 poise after 5 minutespreheating and 12,120 poise at 15 minutes, an increase of 3.0% perminute.

EXAMPLE 4

The procedure of Example 3 is repeated, except that only 0.73 weightpercent oleic acid based on DMT is employed. Further, the product isbrought to its final melt viscosity by solid state polymerization. Acontrol sample of PBT, not containing oleic acid, is also brought to itsfinal melt viscosity by solid state polymerization. Both samples arecompounded into the flame retardant composition as in Example 3. Themelt viscosity of the composition from oleic acid-modified PBT resin is7110 after 5 minutes and 5300 after 13 minutes, a decrease of2.6%/minute. The melt viscosity of the control is 8670 poise at 5minutes and 8980 poise after 13 minutes, an increase of 0.4%/minute.

EXAMPLE 5

Resin samples from Example 4, prior to solid state polymerization, arecompounded into a glass fiber reinforced flame retardant composition asfollows:

    ______________________________________                                        Composition         Parts by Weight                                           ______________________________________                                        PBT resin           62.3                                                      Brominated polycarbonate                                                                          16.5                                                      Antimony oxide      6.0                                                       Irganox 1093        0.15                                                      Ferro 904           0.05                                                      Glass fibers        15.0                                                      ______________________________________                                    

The melt viscosity of the resin from oleic acid-modified PBT is 4000.The melt viscosity of the control resin is 5000 poise.

EXAMPLE 6

The procedure of Example 3 is repeated on a larger scale. The oleicacid-modified resin provides a composition with 8140 poise meltviscosity. The melt viscosity of the control resin is 12340 poise.

EXAMPLE 7

Poly(1,4-butylene terephthalate) is prepared as in Example 1, butsubstituting 175 g. of a methyl ester of hydrogenated wood rosin("Hercolyn D", Hercules, Inc.) for stearic acid. The product iscompounded into a flame retardant composition as in Example 3. The meltviscosity of the composition, after 5 minutes preheating to 250° C., is10,620 poise and decreases at a rate of 2.5% per minute.

EXAMPLE 8

The procedure of Example 7 is repeated, except that the modifyingadditive consists of 132 g. abietol, the partially hydrogenated alcoholobtained by reduction of wood rosin (Hercules, Inc.). When compoundedinto a flame retardant composition as before, the product has a meltviscosity of 10,200 poise, decreasing at a rate of 2.3% per minute.

EXAMPLE 9

The procedure of Example 7 is repeated, using 180 g. of fullyhydrogenated wood rosin ("Foral AX", Hercules, Inc.). The resultingflame retardant composition has a melt viscosity at 250° C. of 7200poise, decreasing at a rate of 1.0%/minute.

Other modifications are possible. For instance, if the procedure ofExample 1 is repeated, substituting for the 1,4-butanediol and dimethylterephthalate the following materials, respectively:

ethylene glycol and dimethyl terephthalate;

ethylene glycol and (i) dimethyl terephthalate and (ii) dimethylisophthalate at a molar ratio of 70/30 (i): (ii); or

trimethylene glycol and methyl terephthalate, polyester compositionswith stabilized melt viscosity will be obtained.

Obviously, other modifications and variations of the present inventionare possible in the light of the above teachings. For example, insteadof stearic acid, methyl stearate can be used. It is, therefore, to beunderstood that changes may be made in the particular embodiments of theinvention described which are within the full intended scope of theinvention as defined by the appended claims.

We claim:
 1. A flame retardant polyester composition with stable meltviscosity comprising (1) a high molecular weight linear thermoplasticpolyester resin selected from the group consisting of polymeric glycolterephthalate and isophthalate esters having repeating units of thegeneral formula: ##STR4## wherein n is a whole number of from 2 to 10,and mixtures of such esters, and (2) a flame retardant additivecomprising an aromatic (poly-)carbonate, said composition produced bythe process comprising (i) adding a small effective amount of arelatively non-volatile monofunctional ester-forming compound to anesterification mixture comprising the corresponding glycol andterephthalic or isophthalic acid or a reactive derivative thereof andheating the mixture under polyesterification conditions until formationof said melt viscosity stabilized polyester is substantially complete,and (ii) intimately blending the melt viscosity stabilized polyesterresin with said flame retardant additive.
 2. A composition as defined inclaim 1 wherein said polyester is poly(1,4-butylene terephthalate).
 3. Acomposition as defined in claim 1 wherein said non-volatilemonofunctional ester-forming organic compound contains only carbon,hydrogen and oxygen.
 4. A composition as defined in claim 3 wherein themonofunctional ester-forming groups in said organic compound is acarboxyl group, a carboalkoxyl group or a hydroxyl group.
 5. Acomposition as defined in claim 4 wherein said organic compound isstearic acid.
 6. A composition as defined in claim 4 wherein saidorganic compound is oleic acid.
 7. A composition as defined in claim 4wherein said organic compound is a methyl ester of hydrogenated woodrosin.
 8. A composition as defined in claim 4 wherein said organiccompound is a partially hydrogenated alcohol prepared by reduction ofwood rosin.
 9. A composition as defined in claim 4 wherein said organiccompound is a fully hydrogenated wood rosin.
 10. A composition asdefined in claim 1 wherein said monofunctional ester-forming organiccompound is present in an amount of from about 0.1 to about 15% byweight based on the total weight of said mixture.
 11. A composition asdefined in claim 10 wherein said monofunctional ester-forming organiccompound is present in an amount of from about 0.5 to about 12% byweight based on the total weight of said mixture.